PROJECT SUMMARY/ABSTRACT Inheritance of Apolipoprotein E (ApoE) gene variant APOE-?4 is the strongest genetic risk factor for Alzheimer?s disease (AD), a rapidly growing burden of health care for the aging United States. There is a fundamental gap in understanding how expression of APOE-?4, which encodes ApoE isoform ApoE4, influences neuronal function and contributes to AD pathogenesis. This gap, until filled, represents our insufficiency in understanding AD and our inability to provide treatment and prevention to AD and risk carriers. A central event in AD pathogenesis is deposition of amyloid-? (A?) peptide, generated from a series of protease cleavages of amyloid precursor protein (APP). Using pure human neurons derived from human embryonic stem cells or iPSCs, the applicant has recently identified a signaling pathway by which ApoE4 stimulates APP transcription and consequently induces A? production in neurons, more effectively than the other two isoforms ApoE2 and ApoE3. This pathway hinges on double leucine-zipper kinase DLK and downstream MAPK signaling, and is activated by three ApoE isoforms differentially in the potency rank order of ApoE4>ApoE3>ApoE2, paralleling AD risk rank order. Given this striking ApoE isoform-specific effect, the overall objective of this application is to determine whether and how ApoE4-specific activation of DLK/MAPK signaling pathway may account for ApoE4?s deleterious AD-promoting effect. My central hypothesis is that ApoE4 may predispose to AD by stimulating DLK/MAPK and increasing neuronal A? production chronically, and that interference with ApoE-enhanced APP transcription via manipulations of DLK/MAPK pathway may delay AD pathogenesis. I plan to test my central hypothesis, thereby accomplishing the overall objective for this project by pursuing these following specific aims: 1) Determine whether DLK/MAPK pathway mediates differential APP/A? induction by ApoE isoforms, ApoE4>ApoE3>ApoE2, 2) Identify the transcription control elements of APP gene required for ApoE stimulation, and 3) Determine the functional output of DLK/MAPK pathway in vivo. A combination of genetic and pharmacological manipulations will be used to enhance or inhibit ApoE-activated DLK/MAPK pathway in cultured neurons and in animal brains. The molecular mechanism and behavioral significance of this pathway will be rigorously examined. The overall approach is innovative because it departs from the status quo by utilizing human neurons and focusing on the underappreciated mechanisms of APP transcription in light of AD pathogenesis. The proposed research is significant, because it is expected to define a new role of ApoE in the brain, and also to explore new therapeutic horizons by targeting DLK-MAPK pathway as an alternative strategy. Findings will advance greatly our knowledge of AD pathogenesis, and have strong potential for future translation into urgently needed treatment and prevention.